Zone Ophiolites Research Articles

Compositionally heterogeneous podiform chromitite in the Shetland Ophiolite Complex (Scotland): Implications for chromitite petrogenesis and late-stage alteration in the upper mantle portion of a supra-subduction zone ophiolite

The mantle sequence of the ~492Ma Shetland Ophiolite Complex (SOC; Scotland) contains abundant compositionally heterogeneous podiform chromitite bodies enclosed in elongate dunite lenses in the vicinity of the petrological Moho. Chromitite petrogenesis and late-stage alteration events recorded in these seams are examined here using petrography, mineral chemistry and crystal structural data. The resistant nature of Cr-spinel to serpentinisation and other late-stage alteration means that primary igneous compositions are preserved in unaltered crystal cores. Chromitite mineralogy and texture from five sampled localities at The Viels, Hagdale, Harold's Grave, Nikka Vord and Cliff reveal significant inter-pod chemical heterogeneity. The Cr-spinel mineral chemistry is consistent with supra-subduction zone melt extraction from the SOC peridotites. The occurrence of chromitite seams in the centres of the dunite lenses combined with variable Cr-spinel compositions at different chromitite seam localities supports a model of chromitite formation from spatially (and temporally?) fluctuating amounts of melt–rock interaction through channelised and/or porous melt flow.Pervasive serpentinisation of the SOC has led to the almost complete replacement of the primary (mantle) silicate mineral assemblages with serpentine (lizardite with minor chrysotile and antigorite). Magmatic sulphide (e.g., pentlandite) in dunite and chromitite is locally converted to reduced Ni-sulphide varieties (e.g., heazlewoodite and millerite). A post-serpentinisation (prograde) oxidisation event is recorded in the extensively altered Cliff chromitite seams in the west of the studied area, where chromitite Cr-spinel is extensively altered to ferritchromit. The ferritchromit may comprise >50% of the volume of the Cliff Cr-spinels and contain appreciable quantities of 1–2μm inclusions of sperrylite (PtAs2) and Ni-arsenide, signifying the coeval formation of these minerals with ferritchromit at temperatures of up to ~500°C. The SOC chromitite Cr-spinels thus not only preserve key insights into the complex melting processes occurring in the upper mantle wedge but can also be utilised to construct a comprehensive alteration history of the lower mantle portions of such supra-subduction zone ophiolites.

Geochemistry and petrogenesis of the Late Cretaceous Haji‐Abad ophiolite (Outer Zagros Ophiolite Belt, Iran): implications for geodynamics of the Bitlis–Zagros suture zone

The Haji‐Abad ophiolite in SW Iran (Outer Zagros Ophiolite Belt) is a remnant of the Late Cretaceous supra‐subduction zone ophiolites along the Bitlis–Zagros suture zone of southern Tethys. These ophiolites are coeval in age with the Late Cretaceous peri‐Arabian ophiolite belt including the Troodos (Cyprus), Kizildag (Turkey), Baer‐Bassit (Syria) and Semail (Oman) in the eastern Mediterranean region, as well as other Late Cretaceous Zagros ophiolites. Mantle tectonites constitute the main lithology of the Haji‐Abad ophiolite and are mostly lherzolites, depleted harzburgite with widespread residual and foliated/discordant dunite lenses. Podiform chromitites are common and are typically enveloped by thin dunitic haloes. Harzburgitic spinels are geochemically characterized by low and/or high Cr number, showing tendency to plot both in depleted abyssal and fore‐arc peridotites fields. Lherzolites are less refractory with slightly higher bulk REE contents and characterized by 7–12% partial melting of a spinel lherzolitic source whereas depleted harzburgites have very low abundances of REE and represented by more than 17% partial melting. The Haji‐Abad ophiolite crustal sequences are characterized by ultramafic cumulates and volcanic rocks. The volcanic rocks comprise pillow lavas and massive lava flows with basaltic to more‐evolved dacitic composition. The geochemistry and petrology of the Haji‐Abad volcanic rocks show a magmatic progression from early‐erupted E‐MORB‐type pillow lavas to late‐stages boninitic lavas. The E‐MORB‐type lavas have LREE‐enriched patterns without (or with slight) depletion in Nb–Ta. Boninitic lavas are highly depleted in bulk REEs and are represented by strong LREE‐depleted patterns and Nb–Ta negative anomalies. Tonalitic and plagiogranitic intrusions of small size, with calc‐alkaline signature, are common in the ophiolite complex. The Late Cretaceous Tethyan ophiolites like those at the Troodos, eastern Mediterranean, Oman and Zagros show similar ages and geochemical signatures, suggesting widespread supra‐subduction zone magmatism in all Neotethyan ophiolites during the Late Cretaceous. The geochemical patterns of the Haji‐Abad ophiolites as well as those of other Late Cretaceous Tethyan ophiolites, reflect a fore‐arc tectonic setting for the generation of the magmatic rocks in the southern branch of Neotethys during the Late Cretaceous. Copyright © 2012 John Wiley & Sons, Ltd.

Determination of stress orientation in Sabzevar Ophiolite Zone in (Khorasan Razavi Province, Iran)

Determination of stress orientation in Sabzevar Ophiolite Zone in (Khorasan Razavi Province, Iran)

Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone

Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This “subduction factory” is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones.The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source:Cwr,add=[Ccpx-obs/[[Dcpx/(Dbulk-PF)]∗[1-(PF/Dbulk)](1/P)]]-[C0,wr]where Cwr,add=concentration of FME added to mantle wedge during a given melt increment, Ccpx-obs=concentration of observed pyroxene, Dcpx and Dbulk=mineral and bulk partition coefficients, P=melt proportion, and F=melt fraction required to model the observed MREE–HREE concentrations. Application of this algorithm to CRO peridotites shows that fluid influx must be continuous with open system melting, which allows us to calculate FME concentrations for small melt increments. Addition of the calculated FME concentrations to depleted MORB mantle (DMM) asthenosphere or refractory arc mantle (RAM) results in pooled magmas that match primitive arc tholeiites and boninites.

Petrology of mantle peridotites and volcanic rocks of the narrowest Mesozoic ophiolitic zone from Central Iran (Surk area, Yazd province)

Petrology of mantle peridotites and volcanic rocks of the narrowest Mesozoic ophiolitic zone from Central Iran (Surk area, Yazd province)

New interpretation of tectonic model in south Tibet

We present a new interpretation of tectonic evolution in southern Tibet that is drastically different from the existing models. A detailed tectono-sedimentary study crossing the Yarlung Zangpo ophiolite zone shows that many geological features are different from those commonly described in large subduction collision models. For example, no N–S oriented shear zones are found between the ophiolitic sequence and country flysch strata, whereas a conformable contact relationship is recognized between them. A tectonic window exists inside the ophiolite body in the Bailang region. Some intrusion-like mafic–ultramafic bodies occurred in the Renbu region, where the country strata are in sub-concordant contact with these bodies and show contact metamorphic aureole. Toward the west, the ophiolite zone was separated by flysch sequences into sub-parallel branches. In the Lhasa region, the sedimentary facies are similar on both sides of the Zangpo Valley, and have preserved an intact Mesozoic basin system. Instead of ophiolite rocks, volcaniclastic deposits occurred in the corresponding location of the ophiolite in the Zangpo Valley. Consequently, we conclude that the Zangpo ophiolite zone has a tectonic affinity of back-arc basin with its spasmodic growth of juvenile oceanic crust. The real tectonic suture, or the closure zone of the Neotethys, should be represented by the High Himalaya Central Gneiss Unit, which shows a large scale strong shearing in same orientation, high metamorphism and protracted re-mobilization. The oceanic crust subducted northward and split off the Himalaya continental front arc, created the Zangpo back-arc basin since Late Triassic. The collapse of the Zangpo back-arc basin by supra-subduction occurred since the Eocene. The final collision between India and the Himalayan arc took place since Late Eocene with a re-mobilized large shear system. The major mylonitic zones migrated progressively southward with bulk of shear slip absorbing the crust of north India and south Tibet.

The formation and evolution of the Bitincke nickel laterite deposit, Albania

The Bitincke nickel laterite deposit, located in the south east of Albania, contains an estimated ore resource of 35.6 Mt of nickel ore with a grade of 1.2 % Ni. The deposit developed on peridotites within Late Jurassic ophiolites which were obducted in the Early Cretaceous and which form part of the Albanian Mirdita ophiolite zone. Limestones and conglomerates overlying the deposit delimit the minimum age of lateritization to the mid-Eocene. The laterite is composed of two distinct ore zones characterized by silicate nickel and iron oxide phases. Within the silicate zone, Ni concentrations reach a maximum of 1.5 wt% and although laterally and vertically variable this zone is typically characterized by olivine and serpentine at the base of the horizon which are gradually replaced by secondary silicates and iron oxides up section. The boundary between the silicate and the oxide zone above is normally sharp and characterized by an increase in Fe2O3 (from ~10 to 80 wt%), decreases in SiO2 (from ~30 to 5 wt%) and a dramatic reduction in MgO content (from ~10 to 0.1 wt%). The oxide horizon is dominated by goethite and displays relatively little variation in textural morphology or geochemistry. Nickel concentrations are greatest near the base of this zone, reaching a maximum of 2.3 wt%. Weathering profile formation and variation in the thickness of the deposit was controlled by the interaction between topography, faulting and protolith fracture density. The oxide zone formed on topographic highs was subject to increased rates of erosion, whereas the laterite profile within topographic lows, and in areas of relatively high fracture density, tends to be thicker due to increased permeability. The most substantive sections of the Ni laterite weathering profile developed in small fault controlled basins and were preserved by the deposition of a sequence of limestones and mudstones.

Onset of an Aptian carbonate platform overlying a Middle-Late Jurassic radiolaritic-ophiolithic mélange in the Mirdita Zone of Albania

Late Jurassic and Early Cretaceous carbonate sediments of different age and facies, form the post-emplacement settings on top of the Mirdita Ophiolite Zone in northern Albania. They seal the early Late Jurassic emplacement of the ophiolite knappes, are the only datable relics overlying the ophiolites in the Late Jurassic - Early Cretaceous, aiding the evaluation of the different tectonic movements associated with uplift and erosion as well as sea-level fluctuations during that time span. These carbonates overlie ophiolite-derived clastics very often as shallowing-upward sequences. One of the largest shallow-water platforms is the Mali I Shenjtit platform, previously assigned to the Barremian-Aptian. A well-preserved section at the base of the platform can be ascribed to the Early-Late Aptian based on orbitolinid foraminifera together with dasycladalean algae. This carbonate platform is the youngest preserved Early Cretaceous platform in Albania, but was largely eroded away. Late Cretaceous shallow-water rudist limestones are widespread and preserved in the Dinaride-Albanide-Hellenide realm directly above an eroded older basement. Therefore, this Aptian platform is very important for the reconstruction of the Early Cretaceous palaeogeography in the northwestern Neotethyan realm. In addition, the stratigraphic ranges of characteristic Aptian orbitolinid foraminifera, Rectodictyoconus giganteus SCHROEDER and Mesorbitolina texana (ROEMER), are discussed on the basis of these results.

Correlation of metabasic rocks from metamorphic soles of the Dinaridic and the Western Vardar zone ophiolites (Serbia): Three contrasting pressure-temperature-time paths

The field, petrological-mineralogical, geochemical and geochronological data of the metamorphic sole rocks recorded beneath the Fruska Gora, Povlen (Tejici), Stolovi and Banjska ophiolites in the Western Vardar Zone (WVZ) and beneath the Zlatibor, Bistrica, Sjenicki Ozren and Brezovica ophiolites in the Dinaridic ophiolite belt (DOB) in Serbia are compared. The focus has been made on metabasic rocks formed in contact with the oceanic crust members: cumulate gabbro and basalts of SSZ-type with E-MORB and OIB-signature and more evolved tholeiitic basalts of MOR-affinity. Amphibole, the major phase formed from the mafic sole components, depending on pressure-temperature conditions exhibits compositional variations. According to mineral assemblages, estimated P-T conditions and ages, the potential P-T paths are given: high pressure - low temperature blueschist facies assemblage (7-9 kbar and ~400?C and <300-350?C and 4-8 kbar), recorded only in the metamorphic sole at the Fruska Gora (WVZ); high pressure - high temperature amphibolite to granulite facies (8-10 kbar and >700-850?C), recorded in both domains, the WVZ (Banjska) and the DOB (Bistrica, Sjenicki Ozren, Brezovica) and medium pressure - medium temperature amphibolite facies assemblages (~3.5-7 kbar and >350-650?C) recognized in the WZV (Tejici, Devovici) and the DOB (Zlatibor). The peak metamorphic conditions point to depths of the oceanic lithosphere detachment and its initial cooling at 10-30 km, but the ages and tectonic setting of ophiolites remain poorly constrained. The summarized data may be used as an important key in geodynamic evolution of the Mesozoic Tethyan ophiolites.

Preservation of terranes during the assembly of the Annieopsquotch Accretionary Tract: Inferences from the provenance of a Middle Ordovician ophiolite to arc transition, central Newfoundland Appalachians1Geological Survey of Canada Contribution 20100463.2This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Newfoundland Appalachians are a classic area for studying the record of arc development and terrane accretion processes with excellent exposure to different crustal levels that are minimally deformed and metamorphosed. This area also provides a link between the once continuous Appalachian and Caledonian orogens. The Annieopsquotch Accretionary Tract lies along the Red Indian Line, within the peri-Laurentian realm of the central Newfoundland Appalachians. The Darriwilian (468–461 Ma) tectonostratigraphic units of the Annieopsquotch Accretionary Tract are commonly characterized by polymictic volcanogenic conglomerate horizons. A conglomerate horizon at the interface between a suprasubduction zone ophiolite and its calc-alkaline volcanic arc cover sequence is herein investigated for zircon and geochemical provenance. Geochronology revealed a maximum age of deposition of 467 ± 4 Ma with zircon inheritance ranging from ca. 500 to 2800 Ma, consistent with a peri-Laurentian continental basement source. Four types of volcanogenic conglomerate clasts are noted on the basis of lithogeochemistry: arc andesite; calc-alkaline basalt; tholeiitic basalt; and non-arc rhyodacite. Tholeiitic basalt clasts are likely locally derived, perhaps from the underlying Skidder Formation. Other volcanic clasts do not have any known geochemical equivalents in the Annieopsquotch Accretionary Tract and hence appear to be exotic. The dominant zircon population suggests that the exotic clasts were derived from a ca. 467 Ma peri-Laurentian andesitic volcanic arc that once formed part of the Annieopsquotch Accretionary Tract.

Geochemistry of ultramafic rocks and hornblendite veins in the Fiskenæsset layered anorthosite complex, SW Greenland: Evidence for hydrous upper mantle in the Archean

The Fiskenæsset Complex, SW Greenland, contains the world's best preserved Archean (∼2970Ma) layered anorthosite, leucogabbro, gabbro, and ultramafic association. The complex was emplaced into Archean oceanic crust distal from continental lithosphere and later intruded by tonalites, trondhjemites and granodiorites (TTG) constituting Archean continental crust. The complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions.This study presents new whole-rock major and trace element, petrographic, and SEM (Scanning Electron Microscope)-BSE (backscatter electron) image data for a 45–60m-thick ultramafic sill and cross-cutting hornblendite veins. The ultramafic sill is composed of ∼75% olivine–pyroxene hornblendite, ∼15% pyroxene hornblendite, and ∼10% hornblende pyroxenite. Despite granulite facies metamorphism and multiple phases of deformation, the Fiskenæsset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and Alaskan-type ultramafic complexes. Hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism. Formation of orthopyroxene–magnetite symplectitic (vermicular) intergrowths, mainly at the expense of olivine, is attributed to chemical reactions between late stage, residual hydrous melts and olivine.Field, geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion. High MgO (16–31wt.%) contents suggest that the sill was emplaced as a crystal mush, rather than as melt only. Petrographic observations and BSE images indicate that olivine, orthopyroxene, and clinopyroxene were the major crystal phases, whereas hornblende represents the main interstitial liquid phase in the crystal mush. The sill exhibits three different REE patterns, corresponding to three cryptic layers. Although the origin of these REE patterns is not fully understood, they may reflect the intrusion of three batches of magma, containing different hornblende-forming melt/crystal ratios. Geochemical data indicate that the cross-cutting hornblendite veins likely originated from evolved, late-stage melts but were not related to the melts of the ultramafic sill.Large negative Nb anomalies (Nb/Nb*=0.04–0.66) suggest that the Fiskenæsset hornblendites, hornblende pyroxenites, and hornblendite veins were derived from a hydrous sub-arc mantle peridotite. Phanerozoic hornblendites are typically associated with supra-subduction zone ophiolites and magmatic arcs. Accordingly, it is suggested that water was recycled to the source of the Fiskenæsset hornblendites through subduction of altered oceanic crust. Recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites, but also played an important role in the formation of TTG-dominated Archean continental crust. Therefore, the field, petrographic and geochemical data presented in this study suggest that the origin of the Mesoarchean to Neoarchean terrane in the Fiskenæsset region, SW Greenland, is consistent with subduction zone geodynamic processes, rather than non-uniformitarian, density-driven, vertical, crustal overturn, and diapiric processes.

RADIOLARIAN AGES FOR THE SEDIMENTARY COVER OF SEVAN OPHIOLITE (ARMENIA, LESSER CAUCASUS)

Age data of Armenian ophiolites are of great importance for understanding the palaeogeographic and geodynamic evolution of Tethys in the Caucasus area and for lateral correlations of Tethyan ophiolitic belts in the Middle East area. Moderately well-preserved radiolarian assemblages obtained during this study from the Sarinar section of eastern Armenia allow to investigate the sedimentary and volcanic history recorded in a ca. 30 m-thick radiolarite sequence associated with spilitic lavas of the Sevan ophiolitic suture zone. The latest Bajocian-early Bathonian assemblage yielded from a sample situated close to the base of the studied sequence suggests that radiolarian ooze accumulation took place on oceanic crust of late Middle Jurassic age (probably Bajocian). Several tuffite levels are intercalated within the lower to middle part of the radiolarite sequence; a late Bathonian-early Oxfordian assemblage from a sample taken just below the uppermost tuffitic bed, suggests that during this time the oceanic domain of Sevan ophiolites was close to subaerial volcanic activity.

The Indus–Yarlung Zangbo ophiolites from Nanga Parbat to Namche Barwa syntaxes, southern Tibet: First synthesis of petrology, geochemistry, and geochronology with incidences on geodynamic reconstructions of Neo-Tethys

The purpose of this first synthesis is to summarize findings on the Yarlung Zangbo Suture Zone (YZSZ) ophiolites and discuss still remaining problems. The YZSZ studied for almost 30years and is the youngest of the sutures recognized on Tibet Plateau. It is now acknowledged that the YZSZ is a complex assemblage of sedimentary, metamorphic and igneous rocks created during and shortly after the collision between India and Eurasia. The ages of the various lithologies span a time interval from the Jurassic to the Middle Miocene, with some Permian and Devonian exotic blocks from mélange zone. The YZSZ is characterized by ophiolitic complexes and ophiolitic mélange. The ophiolites are of two types: non dismembered and dismembered sections. The non disturbed sections, although tectonically reworked, are observed along the segment from Dazhuqu to Jiding in Xigaze area and Spontang ophiolite. The dismembered sequences are found in various locations such as Nidar, Kiogar, Jungbwa, Saga, Sangsang, Xigugabu, and Luobusa. The incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins. The ophiolites are distributed into two groups of ages: the Luobusa, Zedang and Kiogar sequences being Jurassic-Lower Cretaceous whereas all other sequences are of Lower Cretaceous age. Compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70My from their initial genesis to obduction which occurred around 70–90My ago. Ophiolites differ in terms of petrological and geochemical aspects however, they were all generated in suprasubduction zone and more specifically in arc (few fore-arc) and back-arc settings. Synthesis of more than 700 geochemical analyses show variable mixing of components from N-MORB-type to IAT-CAB and to OIB end-members. The Jurassic ophiolites show the maximum of arc component while the Lower Cretaceous ones show little to strong mixing. In addition, most ophiolites were created in short lived (30My) basins and generated close to the Eurasiatic continental margin. We propose that Ladakh–Tibet ophiolites were generated in a suprasubduction context similar to Mariana arc, interarc and back-arc or Tonga–Lau system. The variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system.

Mantle ultrabasites of ophiolite complexes in the Polar Urals: Petrogenesis and geodynamic environments

Geological, mineralogical, and geochemical data on ultrabasites in the Polar Ural Ophiolite Zone in the Main Ural Fault zone indicate that the origin of the ultrabasites was controlled by the depletion process during the partial melting of mantle material, fluid-magmatic transformations, and subsequent metamorphism in various geodynamic environments. In a spreading environment above a subduction zone, the lherzolites and harzburgites of the older complex were produced by partial (10–25%) melting. The fluid-magmatic transformations were associated with the massive refertilization and enrichment in LREE, MREE, and incompatible trace elements. Along with this, the ultrabasites were affected by depyroxenization (dunitization) with the generation of a dunite-harzburgite complex in domains and zones of brittle-plastic deformations first in a spreading and then in an island-arc environment. The subsequent displacement of the ultrabasites to a crustal level was associated with their metamorphism without principal changes in their geochemistry.

Eclogite from the Kumon range, Myanmar: Petrology and tectonic implications

This paper presents the first report on the occurrence of eclogite from the Kumon range of the Western province in Myanmar, which is in the southeastern extension of the Himalayan orogenic belt. The eclogite is mainly composed of omphacite, garnet, hornblende/edenite/katophorite/taramite, biotite, quartz, and rutile. The garnet grains in the eclogite usually show textures of barrier reef, atoll, and table reef types, and have a wide compositional range of Alm58–70Sps1–2Prp9–16Grs14–31. Omphacite grains that occur as garnet inclusions and as isolated crystals in the matrix have similar compositions of Jd34–45 and Jd37–44, respectively. Lesser amounts of jadeitic clinopyroxene (Jd21–38), phengite, biotite, albite, and quartz occur in the lagoon of barrier reef and atoll garnet grains. The matrix omphacite is partly replaced by symplectite of sodic clinopyroxene of Jd20–29 and albite. The lower limits of the pressure/temperature during the eclogite stage, which are defined by the assemblage of garnet, omphacite (Jd40–45), and quartz, are 1.2–1.3GPa/530–615°C. The finding of eclogite from Myanmar suggests the possibility of a wide occurrence of high-pressure metamorphic rocks in the ophiolite zone along the southeastern extension of the Indus-Yarlung Zangbo suture in Myanmar and Indochina.

A 3.51 Ga bimodal volcanics‐BIF‐ultramafic succession from Singhbhum Craton: implications for Palaeoarchaean geodynamic processes from the oldest greenstone succession of the Indian subcontinent

The Palaeoarchaean greenstone belt of the southern Iron Ore Group (SIOG) (3.51 Ga) in the Singhbhum Craton, eastern India, includes low‐strained and low‐greenstone grade bimodal volcanics, Banded Iron Formation (BIF) and chromiferous ultramafics as enclaves within tonalite–trondjhemite–granodiorite (TTG) granitoids, collectively referred to as the Singhbhum Granite (3.4 Ga to 3.1 Ga). The succession comprises, from base to top, a lower unit of massive and pillowed basalt conformably overlain by dacitic lava and pyroclastics which in turn is overlain by a major BIF unit. The ultramafics are juxtaposed with the volcanics‐BIF succession along a thrust fault. The lithological association of pillow lava, subaqueous dacitic lava and pyroclastic rocks and BIF collectively, suggests that the entire succession was deposited in a deep‐marine depositional setting. The ash‐poor dacitic volcanic rock succession with evidences of a transition from suppressed‐volatile deep‐water lava flow and pyroclastics to more evolved mass‐flow deposits with increasing trend of subaqueous flow transformation, records a transition from a deep‐water low‐height volcanic chain to a shallower subaqueous eruption in an aggradational volcanic chain. Geochemical proxies from the bimodal volcanics and ultramafics showing enrichment of La/Nb, Th/Nb, Th/La, Ba/La, Pb/Ce, depletion in Nb–Ta relative to neighbouring REE, together with tectonic discrimination criteria using Nb, Y, Zr, Ti compositions, suggest an extending oceanic arc–forearc geodynamic setting similar to many of the Phanerozoic supra‐subduction zone ophiolites where ophiolite development in the extending upper plate in a relatively short time span is facilitated by slab rollback processes. The positive Eu‐anomaly together with high Y/Ho values from the BIFs also suggests their deposition in close proximity to spreading centres that might have developed over a rifted arc. The bimodal volcanic rock‐BIF‐ultramafic succession of the SIOG with evidence of a convergent margin geodynamic setting is an important example for Palaeoarchaean plate tectonic processes operating on Earth. Copyright © 2011 John Wiley & Sons, Ltd.

Composition, sources, and mechanisms of formation of the continental crust of the Lake zone of the Central Asian Caledonides. II. Geochemical and Nd isotope data

Part II of this paper reports geochemical and Nd isotope characteristics of the volcanogenic and siliceous-terrigenous complexes of the Lake zone of the Central Asian Caledonides and associating granitoids of various ages. Geological, geochronological, geochemical, and isotopic data were synthesized with application to the problems of the sources and main mechanisms of continental crust formation and evolution for the Caledonides of the Central Asian orogenic belt. It was found that the juvenile sialic crust of the Lake zone was formed during the Vendian-Cambrian (approximately 570–490 Ma) in an environment of intraoceanic island arcs and oceanic islands from depleted mantle sources with the entrainment of sedimentary crustal materials into subduction zones and owing to the accretion processes of the amalgamation of paleoceanic and island arc complexes and Precambrian microcontinents, which terminated by ∼490 Ma. The source of primary melts for the low-Ti basalts, andesites, and dacites of the Lake zone ophiolites and island arc complexes was mainly the depleted mantle wedge above a subduction zone. In addition, an enriched plume source contributed to the genesis of the high-Ti basalts and gabbroids of oceanic plateaus. The source of terrigenous rocks associating with the volcanics was composed of materials similar in composition to the country rocks at a minor and varying role of ancient crustal materials introduced into the ocean basin owing to the erosion of Precambrian microcontinents. The sedimentary rocks of the accretionary prism were derived by the erosion of mainly juvenile island arc sources with a minor contribution of rocks of the mature continental crust. The island arc and accretion stages of the development of the Lake zone (∼540–590 Ma) were accompanied by the development of high- and low-alumina sodic granitoids through the melting at various depths of depleted mantle reservoirs (metabasites of a subducted oceanic slab and a mantle wedge) and at the base of the island arc at the subordinate role of ancient crustal rocks. The melts of the postaccretion granitoids of the Central Asian Caledonides were derived mainly from the rocks of the juvenile Caledonian crust at an increasing input of an ancient crustal component owing to the tectonic mixing of the rocks of ophiolitic and island arc complexes and microcontinents. The obtained results indicate that the Vendian-Early Paleozoic stage of the evolution of the Central Asian orogenic belt was characterized by the extensive growth of juvenile continental crust and allow us to distinguish a corresponding stage of juvenile crust formation.

EKOLOŠKE I FLORISTIČKE KARAKTERISTIKE ŠUMA BUKVE I JELE NA BAZIČNIM ERUPTIVIMA OFIOLITSKE ZONE U BOSNI

UDK 581.5:630*1(497.6)
 The forests of beech and fir (with spruce), as the most wider spread in Bosnia and Herzegovina (occupy 562.237 ha, i.e. more than 35% of the forest surface in Bosnia and Herzegovina – MATIĆ ET AL. 1971), have been the subject of numerous researches among which the phytocenological research has great significance. However, lots of questions have not been solved regarding some groups of forests in terms of their ecological circumstances, floristic structure, mutual relationships as well as syntaxonomic position. The forest of beech and fir on basic eruptives, gabbro and diabas, in ophiolite zone in Bosnia, belong to this group.
 Ecological circumstances, especially edaphic conditions, in which these forests arise, show some specific qualities in relation to other substratum where the forests of beech and fir (with spruce) are found, which affects the floristic structure and substance of these forests as well as their syntaxonomic classification.

Geotectonic significance of Neoproterozoic amphibolites from the Central Eastern Desert of Egypt: A possible dismembered sub-ophiolitic metamorphic sole

Supra-subduction zone ophiolites in the Egyptian Central Eastern Desert (CED) occur as clusters in its northern (NCEDO) and southern (SCEDO) parts, displaying abundant island arc–boninitic and MORB/island-arc geochemical affinities, respectively. An amphibolite belt, including the investigated massive to slightly foliated Wadi Um Gheig (WUG) amphibolites, is exposed in the southeast most of the NCEDO thrusting over the El Sibai gneissic association and intruded by late- to post-orogenic granitoids and gabbros. The WUG rocks are metamorphosed under epidote amphibolite to common amphibolite facies. The amphiboles are calcic and represented by actinolitic hornblende to magnesio-hornblende in the epidote amphibolites and magnesio- to ferro-hornblende in the amphibolites. Plagioclase composition varies from pure albite (An3–8) in the epidote amphibolites to andesine and labradorite (An36–65) in the amphibolites. The estimated P–T conditions are in favor of their metamorphism under epidote amphibolite (c. 550–600°C and 2–3±1.5kbar) and amphibolite (c. 618–720°C and 3–6±1.5kbar) facies. The peak metamorphic conditions point to a burial depth of c.15–20km.Geochemically, the WUG amphibolites show basaltic to andesitic compositions of tholeiitic affinity. They display LILE-enriched MORB-normalized patterns with negative Nb anomalies characteristic of the subduction-related rocks. However, their chondrite-normalized rare-earth element (REE) patterns vary from LREE-depleted (LaN/YbN=0.29 to 0.49) to LREE-enriched (LaN/YbN=2.97 to 3.74). Few samples show major and trace element contents typical of boninitic rocks, including U-shaped REE pattern. On the standard tectonic discrimination diagrams the WUG amphibolites plot mostly in the island-arc fields with some samples of MORB and boninitic affinities. Greenschist facies metamorphosed NCEDO obviously share these geochemical characteristics, implying formation in the same tectonic environment, i.e. forearc basin. This argues that the WUG amphibolites likely represent remnants of a dismembered metamorphic sole beneath the NCEDO. Their formation possibly involves intra-forearc basin thrusting followed by emplacement of ophiolite as imbricated stack of dismembered thrust slices in an accretionary wedge setting. This revives interests in geotectonic model in which the CED represents a forearc–arc–back-arc system above a southeast-dipping subduction zone.

Discrimination of the age and tectonic setting for magmatic rocks along the Zagros thrust zone, northwest Iran, using the zircon U–Pb age and Sr–Nd isotopes

The Zagros thrust is a suture zone 10–15 km wide and 800 km long between the Arabian and Iranian plates, extending NW–SE in the west of Iran. Along this zone, some dismembered ophiolite, Eocene volcanic and some gabbroic to dioritic bodies crop out. The basaltic rocks have a zircon U–Pb age of 54.6 Ma; some gabbroic to dioritic bodies have ages of 36.75 ± 0.51 Ma and 36.63 ± 0.40 Ma, and they are rich in LILE and LREE relative to the N-MORB. High ( 87Sr/ 86Sr)i and low ( 143Nd/ 144Nd)i ratios relative to the MORB show that the slab fluid had a greater effect in the evolution of the source magma. The geochemical data infers that the basaltic rocks in the western part, Kamyaran basalt, were produced in an active margin such as an island arc. Geochemical features of gabbroic bodies and Oskul basalts indicate back arc basin-like N-MORB activity. Their chemical compositions and initial isotope ratios of 87Sr/ 87Sr and 143Nd/ 144Nd confirm that all the basaltic and gabbroic bodies along the Zagros fault in the study area were originated from the depleted mantle in the Cenozoic. It is possible that the complex was part of the supra subduction zone ophiolite, which cropped out in the collision of Arabian and Iranian plateau, probably in the Miocene or later.